1. Field of the Invention
The present invention relates to a silicon-based thin film, a process for forming a silicon-based thin film, and a semiconductor device having a semiconductor junction which is formed of the silicon-based thin film.
2. Related Background Art
As a process for forming silicon-based thin films, a high frequency plasma technique is one of useful ways of mass-production of the silicon-based thin films from the viewpoint that enlargement of the area and formation at a low temperature are possible, and that a process through-put can be improved.
With solar cells considered as an example of a semiconductor device having a semiconductor junction which is formed of a silicon-based thin film, solar cells using a silicon-based thin film are advantageous in that the energy can be generated using an inexhaustible source and a clean power generation process over conventional energies obtained by fossil fuels. However, it is necessary to reduce a unit price per generated electric power in order to encourage broad use of them. To this end, there has been an important technological challenge to establish a method to improve a deposition rate and efficiency of photoelectric conversion in a high frequency plasma CVD technique. For thin-film transistors which are used to for example drive a matrix of a liquid crystal display device, thin-film transistors having a higher mobility are required to facilitate reduction in size of the device and provide fine images because such transistors ensure a necessary current value for circuit operation and allow fine pixel pitches even when a gate width of the thin-film transistor becomes small.
In photovoltaic devices having a pin junction, it is effective that an i-type semiconductor layer includes a crystalline phase because photodegradation due to the Staebler-Wronski effect specific to amorphous semiconductors can be restricted when the i-type semiconductor layer including the crystalline phase is used as a i-type semiconductor layer which substantially serves as a photo-absorption layer. Crystalline thin-film transistors have an at least double-figure higher mobility than amorphous thin-film transistors. Therefore, it can be expected that TFT properties be improved significantly.
Against the backdrop of the above, various approaches and efforts have been taken in recent years about the techniques that are associated with fast deposition of a silicon-based thin film and associated with formation of silicon-based thin films including a crystalline phase.
As to the high frequency plasma CVD technique performed at a higher deposition rate, Japanese Patent Publication No. 7-105354 discloses that the inventor focused on a relation of a frequency f and a distance d, between a substrate and electrodes, in a range of f between 25 and 150 MHz, wherein f represents a high frequency (MHz) and d represents a distance (cm) between the substrate and the electrodes. It is also disclosed that such methods are preferable in which the ratio f/d is included between 30 and 100 MHz/cm, in particular, with the distance d included between 1 and 3 cm or a pressure included between 0.1 and 0.5 mbar.
In addition, as to a process for manufacturing crystalline silicon-based thin film layers, Japanese Patent Application Laid-Open No. 11-330520 discloses that silicon-based thin film layers can be deposited at a fast rate when they contain a silane-based gas and a hydrogen gas and are produced under conditions where a pressure in a reaction chamber is set to at least 5 Torr and where a distance between a substrate and electrodes is not larger than 1 cm. It is also disclosed that a photoelectric converter using this exhibits high conversion efficiencies.
When the silicon-based thin film is formed on a substrate, a temperature on a deposition surface affects surface diffusion of various active species that contribute to deposition phenomena, a formation density of unbound species formed on the deposition surface, and delamination reaction from the silicon-based thin film. Therefore, the temperature is considered as an important parameter to control the deposition rate and the film properties.
Conventionally, as a method for controlling the temperature on the deposition surface, the substrate is subjected to sufficient pre-heating before formation of the silicon-based thin film. Alternatively, a heat source having a large thermal capacity is used. Thus, the temperature is kept constant on the substrate at a predetermined temperature in the direction of the film thickness. Then, the silicon-based thin film is formed while maintaining the states.
In a method for forming silicon-based thin films on a substrate using a high frequency plasma CVD, it has come to be found that increasing film deposition rate can be done under the conditions in which a high frequency power to be introduced is increased, a distance between a substrate and a high frequency input unit is shortened, and a high frequency power per a plasma discharge space is increased.
However, the above-mentioned way to control the temperature of the substrate has drawbacks in that increasing a film thickness may gradually lower quality of silicon-based thin films, vary quality of an initial area for deposition, or deform a substrate after deposition due to a stress strain.
In order to provide a silicon-based thin film of good performance at a lower cost, the present invention is directed to provide a silicon-based thin film having good properties at a higher deposition rate and provide a semiconductor device including it. Furthermore, the present invention is directed to provide a semiconductor device including the silicon-based thin films that has good adhesion and weather-resisting properties and that can be manufactured in a short tact time.
The present invention provides a process for forming a silicon-based film on a substrate, comprising providing a temperature gradient in the thickness direction of the substrate in the formation of the silicon-based film wherein the temperature gradient is made such that a deposition surface of the substrate has a higher temperature than a backside.
The present invention also provides a silicon-based film formed on a substrate, the substrate having a temperature gradient in the thickness direction thereof in the formation of the silicon-based film, the temperature gradient being formed using a method that achieves a higher temperature on a side of a deposition surface of the substrate than that on a backside.
Furthermore, the present invention provides a semiconductor device having a semiconductor junction on a substrate, the semiconductor junction comprising silicon-based films, wherein at least one of the silicon-based films in the semiconductor device has a temperature gradient in the thickness direction of the substrate, the temperature gradient being formed using a method that achieves a higher temperature on a side of a deposition surface of the substrate than that on a backside.
It is preferable that the temperature gradient C be defined by C=xcex94T/d wherein d represents a thickness of the substrate and xcex94T represents a temperature difference between the deposition surface and the backside of the substrate, and that a value of C be in a range between 500xc2x0 C./m and 100,000xc2x0 C./m, both inclusive. It is also preferable that heat sources be provided on the deposition surface side of the substrate and the backside thereof, the heat sources being used to apply heat to the substrate in the formation of the silicon-based film.
In addition, the present invention provides a process for forming a silicon-based film on a substrate wherein the substrate has a temperature gradient in the thickness direction thereof in the formation of the silicon-based film, and wherein the direction of the temperature gradient is reversed during the formation of the silicon-based film.
The present invention also provides a silicon-based film formed on a substrate, the substrate having a temperature gradient in the thickness direction thereof in the formation of the silicon-based film, the temperature gradient being formed using a method wherein the direction of the temperature gradient is reversed during the formation of the silicon-based film.
The present invention also provides a semiconductor device having a semiconductor junction on a substrate, the semiconductor junction comprising silicon-based films, wherein at least one of the silicon-based films has a temperature gradient in the thickness direction of the substrate, the temperature gradient being formed using a method wherein the direction of the temperature gradient is reversed during the formation of the silicon-based film.
It is preferable that the temperature gradient C be defined by C=xcex94T/d wherein d represents a thickness of the substrate and xcex94T represents a temperature difference between the deposition surface and the backside of the substrate, and that a value of C be varied within a range not larger than 100,000xc2x0 C./m. It is also preferable that in the process where the direction of the temperature gradient is reversed during the formation of the silicon-based film, the temperature gradient C include a range of at least 500xc2x0 C./m in a process where the deposition surface of the substrate has a higher temperature than the backside and in a process where the backside has a higher temperature than the deposition surface.
In the above description, it is preferable that a cooling mechanism be provided on the deposition surface side of the substrate and/or the backside thereof in the formation of the silicon-based film. It is preferable that the temperature of the backside of the substrate be reduced in the course of forming the silicon-based film. It is also preferable that the silicon-based film be a silicon-based film including a crystalline phase. It is also preferable that the silicon-based film including the crystalline phase have a region of which ratio of the diffraction intensity of (220) planes in the crystalline phase is 80% or higher with respect to the total diffraction intensity obtained using an x-ray or an electron beam. It is preferable that the silicon-based film be formed on the substrate loaded in the vacuum vessel, using a high frequency plasma CVD technique that involves introducing a source gas containing hydrogen and at least one of a hydrogenated silicon gas and a fluorinated silicon gas into a vacuum vessel and introducing high frequency waves into a high frequency input unit in the vacuum vessel. It is preferable that the high frequency waves have a frequency between 10 MHz and 10 GHz, both inclusive. In this event, it is more preferably that the high frequency waves have a frequency between 20 MHz and 300 MHz, both inclusive. It is preferable that a distance between the high frequency input unit and the substrate be equal to or larger than 3 mm but not larger than 30 mm. It is preferable that a pressure in forming the silicon-based film be equal to or higher than 100 Pa (0.75 Torr) but not higher than 5,000 Pa (37.5 Torr). It is preferable that a residence time of the source gas in forming the silicon-based film be equal to or longer than 0.01 seconds but not longer than 10 seconds. In this event, it is more preferable that the residence time of the source gas in forming the silicon-based film be equal to or longer than 0.1 seconds but not longer than 3 seconds. It is preferable that a power density in forming the silicon-based film be equal to or higher than 0.01 W/cm3 but not higher than 2 W/cm3. In this event, it is more preferable that the power density in forming the silicon-based film be equal to or higher than 0.1 W/cm3 but not higher than 1 W/cm3. It is preferable that the silicon-based film contains at least one of oxygen atoms, carbon atoms and nitrogen atoms, and that the total amount thereof be equal to or larger than 1.5xc3x971018 atoms/cm3 but not larger than 5.0xc3x971019 atoms/cm3. It is preferable that the silicon-based film contains fluorine atoms in an amount equal to or larger than 1.0xc3x971019 atoms/cm3 but not larger than 2.5xc3x971020 atoms/cm3. It is preferable that the semiconductor device having the semiconductor junction include at least one pair of pin-type semiconductor junctions, the pin-type semiconductor junction comprising of a first electrically conductive semiconductor layer, an i-type semiconductor layer, a second electrically conductive semiconductor layer, which are laminated on top of each other.